Direction of Hurricane Beta Drift in Horizontally Sheared Flows

The impacts of linear environmental shears on beta drift direction are assessed through numerical experiments with a single-layer, primitive equation model. It is found that cyclonic (anticyclonic) shears turn the beta drift more westward (northward) in the Northern Hemisphere. In addition, the longitudinal shear of meridional flows (]V/]x) is much more effective than the meridional shear of zonal flows (]U/]y) in deflection of the beta drift. A theoretical model, the beta gyre dynamic system, describing evolution of the beta gyre amplitude and phase angle is advanced to interpret the numerical model results. In this model, the nonlinear energy transfer from the beta gyres to the primary vortex and higher asymmetric modes was partially parameterized by linear damping. The semi-empirical theory predicts that 1) beta drift direction is independent of the planetary vorticity gradient; 2) in a quiescent environment, the drift angle is primarily determined by the outer azimuthal flows of the vortex; and 3) in a sheared environmental flow, the deflection of beta drift induced by environmental shears depends mainly on the longitudinal shear of meridional flows. The authors show that the environmental shear changes beta drift angle by advection of beta gyre vorticity and planetary vorticity, which affects beta gyre orientation.